Steiner G, Müller M
Institute of Zoology, University of Vienna, Althanstr.14, A-1090 Vienna, Austria.
J Mol Evol. 1996 Jul;43(1):58-70. doi: 10.1007/BF02352300.
Molecular characteristics, especially 18S rDNA sequences, may be of great value for the study of bivalve evolution and its numerous morphological convergencies once the reliability of these data can be evaluated. The analysis of 11 published complete molluscan sequences and two new ones, Arca noae and Atrina pectinata, reveals considerable differences in relative substitution rates. The gastropod and eulamellibranch species have the fastest and Atrina species have the slowest rates. Two methods are used to assess the information contents of the dataset in addition to bootstrap analysis, spectral analysis, and the "pattern of resolved nodes" technique. Tree reconstructions by parsimony, neighbor-joining, and maximum-likelihood differ in regard to the position of the eulamellibranch family Mactridae and of Crassostrea. Although there is a signal for the monophyly of Bivalvia, Mactridae cluster with Gastropoda in most runs, rendering Bivalvia diphyletic. The position of Crassostrea was extremely variable, probably due to the high substitution rate of this species. Atrina roots deeper than Arca in all trees, although a corresponding signal in spectral analysis is absent. Phylogenetic signals among the three pectinid species are low but sufficient to resolve the branching pattern. The tree inferred from the 18S rDNA and from morphological data has Bivalvia monophyletic with a basal polytomy of Mactridae, Crassostrea, and the remaining Pteriomorphia, where Arca branches off before Atrina and the Pectinidae. Argopecten is sister group to the other two pectinids; 18S sequence data will have great impact on our understanding of bivalve phylogeny, but only when more sequences of similar substitution rates are available.
一旦这些数据的可靠性能够得到评估，分子特征，尤其是18S rDNA序列，对于双壳贝类进化及其众多形态趋同现象的研究可能具有重要价值。对11个已发表的完整软体动物序列以及两个新序列（无齿蚶和栉江珧）的分析揭示了相对替换率存在相当大的差异。腹足纲和真瓣鳃纲物种的替换率最快，而江珧属物种的替换率最慢。除了自展分析、频谱分析和“解析节点模式”技术外，还使用了两种方法来评估数据集的信息含量。通过简约法、邻接法和最大似然法进行的系统发育重建在真瓣鳃纲的樱蛤科和牡蛎属的位置上存在差异。尽管有信号表明双壳贝类是单系的，但在大多数分析中樱蛤科与腹足纲聚在一起，使得双壳贝类成为并系的。牡蛎属的位置变化极大，可能是由于该物种的替换率很高。在所有的系统发育树中，江珧属的根基都比蚶属更深，尽管在频谱分析中没有相应的信号。三种扇贝科物种之间的系统发育信号较低，但足以解析分支模式。从18S rDNA和形态学数据推断出的系统发育树中，双壳贝类是单系的，樱蛤科、牡蛎属和其余珍珠贝亚目构成基部多歧分支，其中蚶属在江珧属和扇贝科之前分支。海湾扇贝是另外两种扇贝科物种的姐妹群；18S序列数据将对我们理解双壳贝类系统发育产生重大影响，但前提是要有更多替换率相似的序列。
